Corrects the auth_token middleware configuration

Partially addresses LP 1207504

We need to ensure that auth_uri is set properly, that
auth_version defaults to v2.0, even when the v3 Keystone
API is available, and that the non-auth_uri config options
refer to the admin endpoint, not the service endpoint.

Change-Id: Ifbfbbf86bb6956b9b7c366dec5a8a12b383a476b